A power converter is a power processing circuit that converts an input voltage or current source waveform into a specified output voltage or current waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage waveform into a specified output voltage waveform. A boost converter is one example of a switched-mode power converter that is typically employed in off-line applications wherein power factor correction and total harmonic distortion (THD) reduction at the input and a stable, regulated voltage at the output are desired.
A non-isolated power factor correction (PFC) boost converter generally includes a boost inductor and a power switch coupled to the boost inductor. The boost converter further includes a rectifying diode coupled to a node between the boost inductor and the power switch. The boost converter still further includes an output capacitor coupled across an output of the boost converter. The output capacitor is usually large to ensure a constant output voltage. A load is then connected in parallel across the output capacitor. The output voltage (measured at the load) of the boost converter is always greater than the input voltage.
The boost converter generally operates as follows. The power switch is closed (conducting) for a first interval D (D interval). The rectifying diode is reverse-biased, isolating the output capacitor and, therefore, the load from the input of the boost converter. During this interval, the input voltage supplies energy to charge the boost inductor and the inductor current increases. Since the load is isolated from the input voltage, a stored charge in the output capacitor powers the load. Then, for a second interval 1-D (1-D interval), the power switch is opened (non-conducting). The inductor current decreases as energy from both the boost inductor and the input flows forward through the rectifying diode to charge the output capacitor and power the load. By varying a duty cycle of the power switch, the output voltage of the boost converter may be controlled.
The boost converter may be operated in three modes: continuous conduction mode (CCM), discontinuous conduction mode (DCM) or critical mode (CM). The modes are defined by characteristics of the inductor current. More specifically, in CCM, the inductor current is unidirectional and is always greater than zero. In DCM, the inductor current is unidirectional and is equal to zero for a period of time during each switching cycle. In CM, the inductor current is unidirectional and reaches zero only for an instant during each switching cycle.
As previously mentioned, the boost converter, when employed as a power factor corrector, generally provides adequate power factor correction. The power factor is defined as a ratio of the actual power delivered to the load to a product of the voltage and current at the input of the boost converter. The conventional boost converter, however, cannot directly process the AC power available from the AC line. An input full wave rectifier bridge is required at the input of the boost converter to rectify the AC voltage from the AC line. The rectified AC voltage may then be processed by the boost converter. The rectifier bridge is subject to dissipative losses, particularly at low AC line voltages (e.g., 85 to 100 VAC). Power dissipation in the bridge diodes of the rectifier bridge may be as high as 2 to 3% of the total power processed by the power converter. Further, the rectifier bridge may contribute to electromagnetic interference noise generated by the power converter.
As discussed above, the boost converter also contains its own rectifier circuitry, namely, the rectifying diode coupled between the boost inductor and the output capacitor. The rectifying diode may be subject to conduction losses that reduce the efficiency of the boost converter. The combination of AC line rectification (by the rectifier bridge) and switching frequency rectification (by the rectifying diode of the boost converter) reduces the efficiency of the overall power conversion process. Further, while the boost converter provides adequate power factor correction, the output voltage of the boost converter is necessarily greater than the input voltage. The resulting high output voltage may adversely affect the efficiency of other devices to which the boost converter may be connected.
Accordingly, what is needed in the art is a power converter and a controller and method for operation the power converter that overcomes the deficiencies of the prior art.